Waste treatment apparatus

ABSTRACT

An apparatus for denitrifying waste water including means for: generating a fluidized bed containing denitrifying biota on a particulate carrier, metering a carbon source into the waste water and mechanically removing excess bacterial growth from the carrier at predetermined intervals.

This is a division of application Ser. No. 264,346, filed June 19, 1972,now U.S. Pat. No. 3,846,289.

BACKGROUND OF THE INVENTION

The invention relates to a process for the biological treatment ofliquid wastes using fluidized beds. In particular, it is directed to anapparatus for denitrifying waste water.

Traditionally, sewage treatment plants were designed to remove solids anoxygen-demanding organic material. The plants were not intended toremove algal nutrients such as nitrogen or phosphorus. With the largeamounts of fixed nitrogen in the form of ammonia and nitrates that arebeing introduced into the biosphere by the large scale use of syntheticdetergents and fertilizers, and with the demands man makes on hisenvironment owing to population congestion, there definitely appears tobe an imbalance developing in our ecological system that may have longrange consequences for future generations. Today, municipal wastesgenerally contain from 25 to 50 milligrams of nitrogen per liter, in theform of ammonia, amines, nitrate, nitrate and the like. The presence ofsuch nutrients in natural waters causes fertilization and vegetativegrowth in the form of algal blooms. Such blooms often result inaccelerated eutrophication.

Traditional sewage treatment processes such as the activated sludgeprocess and trickling filtration can produce effluents with high nitrateconcentrations. Further, agricultural run-off contains highconcentration of nitrates. Accordingly, there exists an urgent need toreduce the quantity of nitrates and nitrites in waste water prior toreturning the water to the natural environment.

Denitrification processes conducted on experimental bases generallyinvolve nitrifying the effluent from contemporary secondary treatmentplants to oxidize amines and ammonia to nitrates. The nitrified wastesare then subjected to the action of denitrifying biota which convert thenitrates to nitrites and then to nitrogen gas. The nitrogen gas is thenexhausted from the waste water. A carbon source is present duringdentrification. As the nitrate nitrogen is reduced to the gaseousnitrogen molecule, a carbon source is oxidized to carbon dioxide andcellular material is also formed.

Traditional denitrification processes require an unusually longdetention time, usually ranging from 2 to 4 hours. Such detention timesrequire large and expensive facilities for treatment of industrial ormunicipal sewage.

Certain experimental denitrification processes have employed downflowcolumns or beds. Such downflow beds or packed beds tend to be blocked assolids in the waste water are filtered out and further as attached biotaundergo uncontrolled growth on the substrate stones or sand. Suchblockage causes insurmountable head losses. These losses must berelieved by frequent and impractical back washing.

Generally, upflow expanded beds containing activated carbon have beenemployed for the removal of small amounts of carbon or biochemicaloxygen demand (BOD) that remains after biological treatment orphysical/chemical treatment. Biological denitrification has beenobserved in activated carbon beds operated at a low velocity ofapproach, approximately 5 gallons per minute per square foot of bed.However, up to now, upflow bacterial denitrification has been regardedas an undesirable phenomenon resulting in formation of uncontrolledbiological growth which serves to inhibit or impede the primary functionof the bed, the removal of carbon in waste water. Further, onlyinsignificant or inconsequential quantities of nitrogen have beenremoved from waste water in such processes.

A significant defect in all the prior art experimentation with regard todenitrification of nitrified waste water lies in a failure to removewell over 90% of nitrates, while operating at high flow rates and verylow detention times, without plugging.

As employed in the application the term "waste water" or liquid wasteincludes organic or inorganic liquids or mixtures thereof containingbiologically decomposable contaminants and containing the equivalent ofat least about 15 milligrams per liter of nitrogen in an oxidized form;particularly the nitrate and/or nitrite form. Municipal and industrialwaste waters which have undergone nitrification or contain oxidizednitrogen in the above amounts fall within the above definition of wastewater.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the invention to provide anapparatus for treating waste water to reduce nitrogen content employinghigh flow rates and low detention times while maintaining a high removalefficiency.

It is another object of the invention to efficiently denitrify wastewater employing a fluidized bed while controlling the tendency of thebed particles to enlarge and agglomerate.

It is an additional object to denitrify waste water containingsignificant amounts of suspended solids without effectively reducing theefficiency of the process.

Other objects and advantages will become apparent from the followingdetailed discussion of the invention.

The above and other objects are met in an apparatus for biologicallydenitrifying water containing oxidized nitrogen comprising an elongated,substantially vertically disposed container, a manifold disposed towardsthe bottom of said container to control the passage of watertherethrough, inlet means for said container for receiving water to beprocessed, a fluidized bed of micro-organisms attached to a solidparticulate carrier disposed in said container above said manifold,means for adding a carbon source to said fluidized bed, said fluidizedbed being arranged to receive said water from said manifold andbiologically convert substantially all of the oxidized nitrogen to beremoved from the water to nitrogen gas, water and cellular material,outlet means for said container for continuously withdrawing the soprocessed water and nitrogen gas and means for removing excess cellularmaterial from said particulate carrier.

The above and other objects are met in a process for denitrifying wastewater comprising generating a fluidized bed from denitrifying biotaattached to a solid particulate carrier and the waste water, saidcarrier having a particle size of from about 0.2 to 3 millimeters and aspecific gravity of at least about 1.1; providing sufficient amounts ofcarbon source in said waste water to allow nitrified wastes to beconverted to nitrogen by said biota; maintaining said bed at atemperature sufficient to permit bacterial activity; and mechanicallyremoving excess growth from said particulate carrier at predeterminedintervals.

The term "fluidized bed" as employed herein refers to the flow of asuitable liquid upwardly through a bed of suitably sized, solidparticles at a velocity sufficiently high to buoy the particles, toovercome the influence of gravity, and to impart to them an appearanceof movement within a bed expanded to a greater depth than when no flowis passing through the bed.

As nitrogen in the form of nitrates and/or nitrites is removed fromwaste water passing through the fluidized bed, bacterial growth isenhanced and, if unchecked, the size increases and aggregates ofcarrier-supported biota tend to form thus reducing the surface area and,the efficiency of the column. Further, the particles tend to be reducedin specific gravity as they expand and tend to be carried away from thebed. It is a feature of the present process that at periodic intervalsexcess bacterial growth is removed from the particles, thereby leavingsufficient growth for denitrification.

The use of a fluidized bed for denitrification also permits waste waterto be treated wherein such water contains substantial amounts ofsuspended matter. Such suspended matter generally readily passes throughthe fluidized bed. Packed beds are subject to plugging by excess growthand by retention of particulate matter contained in waste water. By theabove process it is also possible to efficiently denitrify waste waterat unexpectedly high flow rates and low detention times.

DESCRIPTION OF PREFERRED EMBODIMENTS

While applicable to the treatment of any fluid having oxidized nitrogenand not toxic to denitrifying bacteria, the present process is readilyadapted to augment secondary treatment systems. The liquid effluent fromtrickling filtration plants or activated sludge processes contain avariety of nitrogenous compounds, including ammonia, amines, nitrates,nitrites and the like. Effluents high in ammonia or amines may besubjected to oxidation and/or the use of aerobic bacteria to convertammonia or amines to nitrates or nitrites. For practical applicationsthe waste water to be treated contains at least the equivalent of about15 milligrams per liter of nitrogen in an oxidized state. The wastewater is passed through an upflow expanded or fluidized bed according tothe invention in the presence of denitrifying bacteria, such asPseudomonas. The nitrates and/or nitrites are converted to inertnitrogen gas and/or cellular material.

While the following discussion is primarily directed toward thetreatment of waste by denitrifying bacteria, including facultativeand/or anaerobic bacteria, it should be recognized that the process isapplicable to the treatment of waste in the presence of oxygen and/oraerobic bacteria, such as Nitrosomonas or Nitrobacter. In such treatmentwaste water containing amines or ammonia may be nitrified and thenitrified wastes thereafter passed into the denitrifying systems. Ingeneral, operation of the nitrifying fluidized bed will parallel that ofthe denitrifying fluidized bed with appropriate obvious modifications.For example, a separate carbon source need not be added to the wastewater prior to nitrification.

The fluidized bed through which the influent waste water is passed ispreferably contained in an upright cylindrical column. Waste waterenters the column through a distribution manifold in its base. Acylindrical manifold plate having a series of spaced apart holes may beemployed in order to regulate and even the flow of waste water throughthe column although a wide assortment of conventional distributiondevices and systems are utilizable.

To perform the process a fluidized bed formed from denitrifying biotaattached to a solid particulate carrier or substrate is generated.Suitable carrier materials include natural or artificial materials suchas coal, volcanic cinders, glass or plastic beads, sand, alumina and,most preferably, activated carbon particles. The size of the particlesis a function of both their specific gravity and surface area. For themost part it is preferred to employ carrier particles between about 0.2and 3 millimeters in diameter. In particular, it is most preferable toemploy porous particles between 0.6 and 1.7 millimeters in diameter(assuming spherical particles). Most preferably, the particles are of auniform size. While the above bed carrier materials are illustrative ofthe preferred bed carrier materials which are useful, other materialsnontoxic to the bacteria, whether natural or synthetic, can be employed.

For optimum denitrification each bed particle preferably has a thinlayer of bacteria seeded thereon. Generally, the bed particles are firstcultured to seed bacteria, such as Pseudomonas, thereon. Seeding isprovided externally or, preferably, internally of the system employingconventional procedures. Common denitrifyers can be present, such asPseudomonas, Bacillus, and/or Micrococcus. The specific gravity of suchseeded particles must be no less than about 1.1 and preferably greaterthan about 1.20 in order to insure that such particles are not carriedout of the system during operation.

In one aspect of operation the bed particles are loaded into an uprightdenitrification column. A carbon source, if needed, is metered into thefeed solution. If the influent feed contains sufficient quantities ofbiologically available organic carbon, then no external carbon sourceneed be employed. The nitrified feed is then pumped into the column at arate sufficient to support the seeded particles in the state offluidization as hereinabove described.

The pressure of the feed at the point of fluidization will varydepending on many factors, including the quantity of bed particles andtheir specific gravity. It has been found that enhanced results areobtained, when the flow rate is from about 6 to 40 gallons per minuteper square foot of natural or artificial bed. Particularly enhancedresults are obtained when the flow rate is from about 8 to 25 gallonsper minute per square foot of bed. Depending upon the flow rateselected, the actual dwell time within a specified column can be aslittle as from three to five minutes.

In a given bed as the flow rate is increased in order to increase thevolume of waste water treated, then a specific bed of biota-attachedparticles will increase in height as the particles separate from eachother. In order to compensate for the tendency of the bed to increase inheight at higher flow rates, it may be described to employ heavieradditional bed particles or employ a new bed having particles of higherspecific gravity. Employing the identical fluidized bed, tests were madewhich showed that as the flow rate was increased from about 12 gallonsper minute per square foot of bed to about 24 gallons per minute persquare foot of bed, the percent expansion of the bed more than doubled.If desired, this effect is counterbalanced by selecting bed particles ofhigher specific gravity when operating at higher flow rates, such assand, garnet, or the like.

In general, the pH of the system is adjusted, if need be, to fall in therange of from about 5.5 to 9.5. Enhanced results are obtained and,accordingly, it is preferred to operate at a pH about 6.5 to 8.5. Thetemperature of the fluidized bed environment should be sufficient topermit bacterial activity. Usually, the bed temperature is kept at fromabout 5° to 45°C. Of course, the temperature will vary with that of theinfluent waste water and, accordingly, ambient operating temperatures inthe order of from 10° to 25°C are satisfactory.

There must be sufficient levels of carbon in the feed influent in orderto provide stoichiometric amounts of carbon to permit oxidized nitrogento be reduced to nitrogen. Of course, if the influent feed contains suchstoichiometric amounts (as set forth hereinafter) of organic carbon,such adjustment may not be necessary. Generally, any inexpensive andreadily available carbon source can be employed. Preferred carbonsources include starch, glucose and, most preferably, methanol. Thecarbon source is added to the influent feed prior to denitrification.Where the carbon source is methanol it has been postulated that thefollowing denitrification reaction occurs:

    NO.sub.3.sup.- + 5/6 CH.sub.3 OH → 1/2 N.sub.2 + 5/6 CO.sub.2 + 7/6 H.sub.2 O +OH.sup.-

sufficient carbon must be present to satisfy this stoichiometric minimumcalculated in light of the amounts of nitrate nitrogen or an equivalentin the feed plus the quantity of carbon required for growth of newmicroorganisms and that required to biologically reduce the dissolvedoxygen present in the influent. Generally 2.5 to 3 milligrams ofmethanol are required per mg. nitrate nitrogen removed.

As the denitrification reaction proceeds in the expanded bed, bacteriatend to grow on the surface of the carrier particles. After a time, ifunchecked, bed particles tend to form thick layers and expand to theextent that they form agglomerates, and/or gelatinous masses. Shouldthis be permitted to occur, then the surface area available fordenitrification is greatly reduced and the efficiency of the process iscorrespondingly reduced. Further, agglomerates tend to be carried out ofthe expanded bed as their specific gravity decreases. They also tend toentrap or become attached to gas bubbles, such as those from thenitrogen gas liberated by the denitrification reaction. The gas bubblesreduce the specific gravity of the agglomerates and tend to carry themaway from the bed toward the top of the column where they can collect asan undesirable floc or leave the system.

In order to overcome these problems excess bacterial growth ismechanically removed from the particles. Sufficient growth in the formof a thin layer of bacteria must remain on the particles in order topreserve the efficiency of the process. Removing all growth, which issuggested for upflow expanded bed processes used for treating wastewater to remove carbon, destroys the efficiency of the present process.To remove the growth, predetermined quantities of bed particles may beremoved from the column by a valve-controlled outlet port andmechanically agitated and abraded to remove excess bacteria. Thisoperation may be performed in a separate abrasion vessel employing amixer which resembles the rotating knife in a Waring Blender. Theabraded particles are then returned to the bottom of the fluidized bed.Alternately, the particles in the abrasion vessel are subjected to theaction of compressed air or water sprays to remove excess biota.

Other suitable agitation mechanisms and apparatus will be apparent tothose skilled in the art. After treatment, the abraded particles aremetered into the expanded bed at its base by suitable inlet port. Thewithdrawal of measured amounts of bed particles, their cleaning andrecycling into the process can be accomplished without significantinterference with the continuity of the process.

In a second and more preferred embodiment, the particles are treated insitu in order to remove excess bacteria from their outer surfaces. Thistreatment can also serve to separate nitrogen gas bubbles formed in thebed and thus reduce loss particles from the bed. Compressed air ispreferably directed through the bed, although a variety of mechanicalagitation apparatus can be employed alone or in combination within thecolumn. For example, mechanical mixers, baffle plates and otherabrasion-type surfaces, water jets directed upwardly and sidewardlyagainst the column walls to create vortices and the like, as well asother suitable conventional agitating means can be employed within thecolumn.

It has been found that sufficient growth is removed, when the height ofthe expanded bed after treatment is reduced on the order of from about10 to 20% of its original expanded length at the same flow rate. Athighly elevated and reduced flow rates, the height may be somewhat aboveor below the range respectively. For removal of excess growth in situusing the air cleaning method, for example, the flow rate to the columnmay be reduced to about one-fourth normal flow. The bed will settle to anew lower height. During and immediately after abrasion, the removedgrowth particles are carried out of the reactor and exhausted from thesystem. Thereafter, the flow rate may be increased to its normalvelocity.

Depending upon the nature of the waste water treated in the fluidizedbed, it may be necessary to employ more than one column connected inseries for efficient denitrification. For most purposes, however, asingle column will suffice. It has been found practical to employ theeffluent from the first column as the influent feed for a second column,where the concentration of nitrites in the effluent is excessive. In thesecond column such nitrites are further reduced to nitrogen gas. Duringstart-up, it had been found useful to recycle at least a portion of theeffluent treated to the column in order to promote initial growth ofbacteria on the bed carrier particles in situ.

In the accompanying drawing a somewhat preferred embodiment of theprocess is illustrated. Waste water A is introduced into the lowerportion of cylindrical column B through a pressure manifold C in thebase of the column. Biota seeded bed particles are fluidized by thepassage of waste water through the column and form a denitrificationfluidized bed D. Denitrified waste water E is exhausted from the columnafter passage through the denitrifying bed. Selected portions of theeffluent are recycled F as required to the influent waste water feed topromote growth of the biota on the particles. A carbon source G ismetered into the waste water influent in sufficient amounts to satisfythe biological reaction for the reduction of nitrogen, as nitrates, inthe waste water.

The metering of sufficient amounts of a carbon source may be conductedautomatically by providing a conventional nitrogen analyzer which isadapted to periodically sample the influent waste water and determineits oxidized nitrogen content. Provision can be made for metering in acarbon source in response to the output of the nitrogen analyzer alongwith metering control based on the incoming flow.

During denitrification, bacterial growth on the particles is monitoredfrom the bed expansion by a conventional optical device or other type ofsolids sensor H which helps to control excess growth. When bed expansionreaches a certain height whereby it reduces light passing through thecolumn to a specified minimum, the bed particles are subjected to theregeneration by abrasion to remove excess growth.

The following examples are illustrative of the invention and are notlimitative of scope:

EXAMPLE I

In order to demonstrate the feasibility of employing a fluidized bed fordenitrification of waste water containing substantial amounts ofnitrates at elevated flow rates, a pair of biological reactors wereconstructed. The biological reactors consisted of columns formed fromPLEXIGLAS acrylic plastic, each reactor being 12 feet high and 3 inchesin inside diameter. Flow entered a bottom PLEXIGLAS manifold platecontaining 1/8 inch diameter holes. Initially, the columns contained 9feet of 12 × 40 mesh activated carbon, seeded with bacteria associatedwith common sewage.

A synthetically prepared feed was employed. The feed included tap water.Sodium sulfite was continuously fed into the feed and maintained thedissolved oxygen of the feed at close to zero in order to insure theintegrity of the anaerobic process. Varying quantities of sodium nitrateand ammonium chloride, as the nitrogen source, were added.

One reactor was in operation for 6 months and maintained excellentbiological growth. Over 90% nitrogen removal was obtained with influentnitrate-nitrogen concentrations varying from about 17 to 39 milligramsper liter. During the below tabulated test runs, the flow rate of theinfluent was measured at 8.1 gallons per minute per square foot of bed.The temperature of the bed was 26°C.

Three test runs are presented in tabular form. The runs were conductedat 3 day intervals. In the table the concentration of nitrogen is inmilligrams per liter. Both influent (feed) and effluent were measuredfor concentration of nitrate and nitrites.

                                      TABLE                                       __________________________________________________________________________          Nitrate  Nitrite  Total Nitrogen                                                                         % N                                          Run No.                                                                             Feed                                                                              Effluent                                                                           Feed                                                                              Effluent                                                                           Feed                                                                              Effluent                                                                           Removed                                      __________________________________________________________________________    1     31.6                                                                              0.3  0.2 1.9  31.8                                                                              2.2  93                                           2     26.6                                                                              0.2  0.3 1.2  26.9                                                                              1.4  95                                           3     17.8                                                                              0.0  0.3 1.0  18.1                                                                              1.0  94                                           __________________________________________________________________________

The high rate of nitrogen removal at the substantial flow rate of 8.1gallons per minute illustrates the efficiency of fluidized beddenitrification. In the column tests, 9 feet of activated carbon wereemployed. During the test the biological growth was permitted to expandwithout treatment. For evaluation purposes a large quantity of activatedcarbon was removed from the reactor.

After such removal this height of the bed in the column at zero flow was6.4 feet. In use the column expanded to 10.8 feet at the operating flow.

EXAMPLE II

In order to evaluate the efficiency of the process at elevated flowrates the denitrification column set forth in Example I was operated ata flow rate of about 12 gallons per minute per square foot of bed for 5days. The temperature of the column was 24.0°C. The column had expandedabout 78% from its packed state. The average detention time of the wastewater in the column was about 6.4 minutes. Tests indicated that theamount of nitrogen removed from the waste water was 30 milligrams perliter.

EXAMPLE III

In order to remove excess growth and reduce the expansion of the bed thefollowing procedure was employed. Denitrification was accomplishedaccording to the procedures set forth in Example I in column constructedaccording to Example I. After the bed had been operating for about aweek the flow rate was reduced from 8 gallons per minute per square footto about 4 gallons per minute per square foot. The bed settled to a newreduced height. At that time compressed air was introduced into thereactor for a 1 minute contact time. The compressed air agitated the bedparticles sufficiently to remove excess growth. The excess growth wascarried out of the column and exhausted from the system. Sufficientgrowth was removed so that the height of the expanded bed was reduced byabout 15% of its original expanded length.

Waste water was passed through the washed bed at 8 gallons per minuteper square foot of bed. Denitrification efficiency was satisfactory. Adaily 10 second air backwash provides further enhanced results.

Various modifications in the process can be employed. For anaerobicbiological systems, oxygen void gases may be employed to provideadditional flow necessary to enhance expansion or fluidization. Ifdesired, auxiliary mixing equipment or pulsing equipment could beemployed to maintain necessary particle movement and separation ofgaseous bubbles from the carrier within the bed or in the freeboardvolume.

In order to reduce the tendency of the bed particles to agglomerate andprovide increased mixing within the bed, the denitrification column orreactor can be sub-divided into a number of vertical compartments ofsmall cross-sectional size. At elevated flow rates of at least about 15gallons per minute per square foot the waste water is braked by thewalls within the column. This produces a circulation and mixing of thebed particles. The particles tend to descend at the wall and rise in themiddle of the vertical pipes. If desired, further subdivision of thereactor could be accomplished by employing crimped and/or plain plasticsheets.

While certain preferred embodiments have been illustrated hereinabovethe invention is not to be limited except as set forth in the followingclaims:

Wherefore we claim:
 1. Apparatus for biologically denitrifying watercontaining oxidized nitrogen comprising an elongated, substantiallyvertically disposed container, a manifold disposed towards the bottom ofsaid container to control the passage of water therethrough, inlet meansfor said container for receiving water to be processed, a fluidized bedof denitrifying microorganisms attached to a solid particulate carrierdisposed in said container above said manifold, means for adding acarbon source to said fluidized bed, said fluidized bed being arrangedto receive said water from said manifold and biologically convertsubstantially all of the oxidized nitrogen to be removed from the waterto nitrogen gas, water and cellular material, outlet means for saidcontainer for receiving said processed water and at least someparticulate carrier having excess cellular material thereon, means forseparating said processed water from said carrier having excess cellularmaterial in fluid communcation with said outlet means, and processingmeans connected to said separating means for receiving said separatedcarrier having excess cellular material and for effecting separation ofsaid excess cellular material from said particulate carrier. 2.Apparatus according to claim 1 including communication means betweensaid container and said processing means for recycling said particulatecarrier onto said fluidized bed.
 3. Apparatus according to claim 1further comprising means for interconnecting said inlet means for saidcontainer with said outlet means for said container in fluid flowcommunication, valve means for controlling the flow in said means forinterconnecting and pump means for recycling treated water through saidinterconnecting means.
 4. Apparatus according to claim 1 wherein saidparticulate carrier is selected from the group consisting of coal,volcanic cinders, glass, plastic beads, garnet, activated carbon,alumina and sand.
 5. Apparatus for biologically denitrifying watercontaining oxidized nitrogen comprising an elongated, substantiallyvertically disposed container, a manifold disposed towards the bottom ofsaid container to control the passage of water therethrough, inlet meansfor said container for receiving water to be processed, a fluidized bedof denitrifying microorganisms attached to a solid particulate carrierdisposed in said container above said manifold, means for adding acarbon source to said fluidized bed, said fluidized bed being arrangedto receive said water from said manifold and biologically convertsubstantially all of the oxidized nitrogen to be removed from the waterto nitrogen gas, water and cellular material, outlet means for saidcontainer for receiving said processed water and said particulatecarrier having excess cellular material thereon, a vessel disposed influid flow relationship with said outlet means for receiving saidprocessed water and said particulate carrier having excess cellularmaterial thereon, first outlet means in said vessel for receiving saidprocessed water, means to effect separation of said excess cellularmaterial from the particulate carrier in fluid flow communication withsaid vessel and fluid flow communication means between the lower portionof said container and said separation means for passing the mixture ofparticulate carrier and excess cellular material back into the fluidizedbed to mix the so separated excess cellular material with the water tobe treated.